Finding the difference between two # values where itemname = the same for one specific id

Question

I have a table called "items"

id       month       year       **itemname**       distmoney
 1        12         2012         chicken           20
 2        12         2012         pork              15
 3        11         2012         chicken           21
 4        11         2012         pork              15

I am trying to find the difference between the "distmoney" for the same itemname between two months FOR ONE SPECIFIC ITEM ONLY (by using WHERE id = $id)

Example:

ID 1, itemname chicken. month 12 distmoney is 20, while month 11 distmoney is 21. I want to be able to calculate the difference of 1 for id=1, itemname=chicken. It should ONLY display the difference for chicken, not any other item names (like pork).

Right now I have the php code to calculate the difference between two numbers, but I am having a tough time figuring out how to grab the previous month distmoney.

Being able to find the previous month's distmoney for a specific itemname and inserting it into a new column called "oldmoney" would also work , but I am not sure to how to do this.

Example: for id 3, itemname chicken. Current distmoney is 20, and the previous month (found in id = 1) is 20. Let's take 20 and insert it into a new column under id 3.

<?php
foreach($rows as $row):
 $number1 = htmlentities($row['distmoney'])
endforeach;

$number1 = $row['distmoney'];
$number2 = ????????; // THIS NEEDS TO BE THE PREVIOUS MONTH DISTMONEY VALUE
if ($number1 <= $number2) {
    $difference = "(Price Lowered) Price difference of $";
    $result = $number2 - $number1;
    $percent = round(100.0*($number2-$number1)/$number1);
    echo $difference; echo $result; echo $percent; echo "%";
} elseif ($number1 > $number2) {
    $result = $number1 - $number2;
    $percent = round(100.0*($number2/$number1-1));
    $addition = "(Price Higher) Price difference of $";
    echo $addition; echo $result; echo $percent; echo "%";
} 

?>

Answer 1

When you query items , ORDER BY itemname first .

Then, do something such as

//Create a variable to hold the "previous month" values
$previousItemName = '';
$previousDistMoney = '';

//Check each record from your query
foreach($rows as $row) {

   //If not first time around && same item name as previous
   if($previousItemName != '' && $previousItemName == $row['itemname']) {

      //subtraction calculation here
      //$difference = $row['distmoney'] - $previousDistMoney;  ??
      //echo $difference;  ??
   }

   //Assign the "previous month" value
   $previousItemName = $row['itemname'];
   $previousDistMoney = $row['distmoney'];
}

Although both of these ways are slightly dirty, you could alternatively do a loop: for($i =0; $i < count($results); $i++) { }

Then check the value of $results[$i-1]['distmoney'];

---

Edit.

If you know the ID from your querystring, simply put that into your query... so that only the 2 'chicken' records are returned:

select  itemname, distmoney from items where itemname in (
   select itemname from items where id = 3
)